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    // Copyright 2015-2016 Brian Smith.
    //
    // Permission to use, copy, modify, and/or distribute this software for any
    // purpose with or without fee is hereby granted, provided that the above
    // copyright notice and this permission notice appear in all copies.
    //
    // THE SOFTWARE IS PROVIDED "AS IS" AND AND THE AUTHORS DISCLAIM ALL WARRANTIES
    // WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
    // MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
    // SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    // WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
    // OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
    // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

    use {bits, der, digest, error, polyfill};
    use super::PUBLIC_KEY_PUBLIC_MODULUS_MAX_LEN;
    use untrusted;

    #[cfg(feature = "rsa_signing")]
    use rand;

    /// Common features of both RSA padding encoding and RSA padding verification.
    pub trait RSAPadding: 'static + Sync + ::private::Private {
        // The digest algorithm used for digesting the message (and maybe for
        // other things).
        fn digest_alg(&self) -> &'static digest::Algorithm;
    }

    /// An RSA signature encoding as described in [RFC 3447 Section 8].
    ///
    /// [RFC 3447 Section 8]: https://tools.ietf.org/html/rfc3447#section-8
    #[cfg(feature = "rsa_signing")]
    pub trait RSAEncoding: RSAPadding {
        #[doc(hidden)]
        fn encode(&self, m_hash: &digest::Digest, m_out: &mut [u8],
                  mod_bits: bits::BitLength, rng: &rand::SecureRandom)
                  -> Result<(), error::Unspecified>;
    }

    /// Verification of an RSA signature encoding as described in
    /// [RFC 3447 Section 8].
    ///
    /// [RFC 3447 Section 8]: https://tools.ietf.org/html/rfc3447#section-8
    pub trait RSAVerification: RSAPadding {
        fn verify(&self, m_hash: &digest::Digest, m: &mut untrusted::Reader,
                  mod_bits: bits::BitLength) -> Result<(), error::Unspecified>;
    }

    /// PKCS#1 1.5 padding as described in [RFC 3447 Section 8.2].
    ///
    /// See "`RSA_PSS_*` Details\" in `ring::signature`'s module-level
    /// documentation for more details.
    ///
    /// [RFC 3447 Section 8.2]: https://tools.ietf.org/html/rfc3447#section-8.2
    pub struct PKCS1 {
        digest_alg: &'static digest::Algorithm,
        digestinfo_prefix: &'static [u8],
    }

    impl ::private::Private for PKCS1 { }

    impl RSAPadding for PKCS1 {
        fn digest_alg(&self) -> &'static digest::Algorithm { self.digest_alg }
    }

    #[cfg(feature ="rsa_signing")]
    impl RSAEncoding for PKCS1 {
        fn encode(&self, m_hash: &digest::Digest, m_out: &mut [u8],
                  _mod_bits: bits::BitLength, _rng: &rand::SecureRandom)
                  -> Result<(), error::Unspecified> {
            pkcs1_encode(&self, m_hash, m_out);
            Ok(())
        }
    }

    impl RSAVerification for PKCS1 {
        fn verify(&self, m_hash: &digest::Digest, m: &mut untrusted::Reader,
                  mod_bits: bits::BitLength) -> Result<(), error::Unspecified> {
            // `mod_bits.as_usize_bytes_rounded_up() <=
            //      PUBLIC_KEY_PUBLIC_MODULUS_MAX_LEN` is ensured by `verify_rsa()`.
            let mut calculated = [0u8; PUBLIC_KEY_PUBLIC_MODULUS_MAX_LEN];
            let calculated =
                &mut calculated[..mod_bits.as_usize_bytes_rounded_up()];
            pkcs1_encode(&self, m_hash, calculated);
            if m.skip_to_end() != polyfill::ref_from_mut_ref(calculated) {
                return Err(error::Unspecified);
            }
            Ok(())
        }
    }

    // Implement padding procedure per EMSA-PKCS1-v1_5,
    // https://tools.ietf.org/html/rfc3447#section-9.2. This is used by both
    // verification and signing so it needs to be able to handle moduli of the
    // minimum and maximum sizes for both operations.
    fn pkcs1_encode(pkcs1: &PKCS1, m_hash: &digest::Digest, m_out: &mut [u8]) {
        let em = m_out;

        let digest_len =
            pkcs1.digestinfo_prefix.len() + pkcs1.digest_alg.output_len;

        // The specification requires at least 8 bytes of padding. Since we
        // disallow keys smaller than 2048 bits, this should always be true.
        assert!(em.len() >= digest_len + 11);
        let pad_len = em.len() - digest_len - 3;
        em[0] = 0;
        em[1] = 1;
        for i in 0..pad_len {
            em[2 + i] = 0xff;
        }
        em[2 + pad_len] = 0;

        let (digest_prefix, digest_dst) = em[3 + pad_len..]
            .split_at_mut(pkcs1.digestinfo_prefix.len());
        digest_prefix.copy_from_slice(pkcs1.digestinfo_prefix);
        digest_dst.copy_from_slice(m_hash.as_ref());
    }

    macro_rules! rsa_pkcs1_padding {
        ( $PADDING_ALGORITHM:ident, $digest_alg:expr, $digestinfo_prefix:expr,
          $doc_str:expr ) => {
            #[doc=$doc_str]
            /// Feature: `rsa_signing`.
            pub static $PADDING_ALGORITHM: PKCS1 = PKCS1 {
                digest_alg: $digest_alg,
                digestinfo_prefix: $digestinfo_prefix,
            };
        }
    }

    rsa_pkcs1_padding!(RSA_PKCS1_SHA1, &digest::SHA1,
                       &SHA1_PKCS1_DIGESTINFO_PREFIX,
                       "PKCS#1 1.5 padding using SHA-1 for RSA signatures.");
    rsa_pkcs1_padding!(RSA_PKCS1_SHA256, &digest::SHA256,
                       &SHA256_PKCS1_DIGESTINFO_PREFIX,
                       "PKCS#1 1.5 padding using SHA-256 for RSA signatures.");
    rsa_pkcs1_padding!(RSA_PKCS1_SHA384, &digest::SHA384,
                       &SHA384_PKCS1_DIGESTINFO_PREFIX,
                       "PKCS#1 1.5 padding using SHA-384 for RSA signatures.");
    rsa_pkcs1_padding!(RSA_PKCS1_SHA512, &digest::SHA512,
                       &SHA512_PKCS1_DIGESTINFO_PREFIX,
                       "PKCS#1 1.5 padding using SHA-512 for RSA signatures.");

    macro_rules! pkcs1_digestinfo_prefix {
        ( $name:ident, $digest_len:expr, $digest_oid_len:expr,
          [ $( $digest_oid:expr ),* ] ) => {
            static $name: [u8; 2 + 8 + $digest_oid_len] = [
                der::Tag::Sequence as u8, 8 + $digest_oid_len + $digest_len,
                    der::Tag::Sequence as u8, 2 + $digest_oid_len + 2,
                        der::Tag::OID as u8, $digest_oid_len, $( $digest_oid ),*,
                        der::Tag::Null as u8, 0,
                    der::Tag::OctetString as u8, $digest_len,
            ];
        }
    }

    pkcs1_digestinfo_prefix!(
        SHA1_PKCS1_DIGESTINFO_PREFIX, 20, 5, [ 0x2b, 0x0e, 0x03, 0x02, 0x1a ]);

    pkcs1_digestinfo_prefix!(
        SHA256_PKCS1_DIGESTINFO_PREFIX, 32, 9,
        [ 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01 ]);

    pkcs1_digestinfo_prefix!(
        SHA384_PKCS1_DIGESTINFO_PREFIX, 48, 9,
        [ 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02 ]);

    pkcs1_digestinfo_prefix!(
        SHA512_PKCS1_DIGESTINFO_PREFIX, 64, 9,
        [ 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03 ]);

    /// RSA PSS padding as described in [RFC 3447 Section 8.1].
    ///
    /// See "`RSA_PSS_*` Details\" in `ring::signature`'s module-level
    /// documentation for more details.
    ///
    /// [RFC 3447 Section 8.1]: https://tools.ietf.org/html/rfc3447#section-8.1
    pub struct PSS {
        digest_alg: &'static digest::Algorithm,
    }

    impl ::private::Private for PSS { }

    #[cfg(feature = "rsa_signing")]
    // Maximum supported length of the salt in bytes.
    // In practice, this is constrained by the maximum digest length.
    const MAX_SALT_LEN: usize = digest::MAX_OUTPUT_LEN;

    impl RSAPadding for PSS {
        fn digest_alg(&self) -> &'static digest::Algorithm { self.digest_alg }
    }

    #[cfg(feature = "rsa_signing")]
    impl RSAEncoding for PSS {
        // Implement padding procedure per EMSA-PSS,
        // https://tools.ietf.org/html/rfc3447#section-9.1.
        fn encode(&self, m_hash: &digest::Digest, m_out: &mut [u8],
                  mod_bits: bits::BitLength, rng: &rand::SecureRandom)
                  -> Result<(), error::Unspecified> {
            let metrics = PSSMetrics::new(self.digest_alg, mod_bits)?;

            // The `m_out` this function fills is the big-endian-encoded value of `m`
            // from the specification, padded to `k` bytes, where `k` is the length
            // in bytes of the public modulus. The spec says "Note that emLen will
            // be one less than k if modBits - 1 is divisible by 8 and equal to k
            // otherwise." In other words we might need to prefix `em` with a
            // leading zero byte to form a correct value of `m`.
            let em = if metrics.top_byte_mask == 0xff {
                m_out[0] = 0;
                &mut m_out[1..]
            } else {
                m_out
            };
            assert_eq!(em.len(), metrics.em_len);

            // Steps 1 and 2 are done by the caller to produce `m_hash`.

            // Step 3 is done by `PSSMetrics::new()` above.

            // Step 4.
            let mut salt = [0u8; MAX_SALT_LEN];
            let salt = &mut salt[..metrics.s_len];
            rng.fill(salt)?;

            // Step 5 and 6.
            let h_hash = pss_digest(self.digest_alg, m_hash, salt);

            // Re-order steps 7, 8, 9 and 10 so that we first output the db mask
            // into `em`, and then XOR the value of db.

            // Step 9. First output the mask into the out buffer.
            let (mut masked_db, mut digest_terminator) =
                em.split_at_mut(metrics.db_len);
            mgf1(self.digest_alg, h_hash.as_ref(), &mut masked_db)?;

            {
                // Steps 7.
                let masked_db = masked_db.into_iter();
                // `PS` is all zero bytes, so skipping `ps_len` bytes is equivalent
                // to XORing `PS` onto `db`.
                let mut masked_db = masked_db.skip(metrics.ps_len);

                // Step 8.
                *(masked_db.next().ok_or(error::Unspecified)?) ^= 0x01;

                // Step 10.
                for (masked_db_b, salt_b) in masked_db.zip(salt) {
                    *masked_db_b ^= *salt_b;
                }
            }

            // Step 11.
            masked_db[0] &= metrics.top_byte_mask;

            // Step 12.
            digest_terminator[..metrics.h_len].copy_from_slice(h_hash.as_ref());
            digest_terminator[metrics.h_len] = 0xbc;

            Ok(())
        }
    }

    impl RSAVerification for PSS {
        // RSASSA-PSS-VERIFY from https://tools.ietf.org/html/rfc3447#section-8.1.2
        // where steps 1, 2(a), and 2(b) have been done for us.
        fn verify(&self, m_hash: &digest::Digest, m: &mut untrusted::Reader,
                  mod_bits: bits::BitLength) -> Result<(), error::Unspecified> {
            let metrics = PSSMetrics::new(self.digest_alg, mod_bits)?;

            // RSASSA-PSS-VERIFY Step 2(c). The `m` this function is given is the
            // big-endian-encoded value of `m` from the specification, padded to
            // `k` bytes, where `k` is the length in bytes of the public modulus.
            // The spec. says "Note that emLen will be one less than k if
            // modBits - 1 is divisible by 8 and equal to k otherwise," where `k`
            // is the length in octets of the RSA public modulus `n`. In other
            // words, `em` might have an extra leading zero byte that we need to
            // strip before we start the PSS decoding steps which is an artifact of
            // the `Verification` interface.
            if metrics.top_byte_mask == 0xff {
                if m.read_byte()? != 0 {
                    return Err(error::Unspecified);
                }
            };
            let em = m;

            // The rest of this function is EMSA-PSS-VERIFY from
            // https://tools.ietf.org/html/rfc3447#section-9.1.2.

            // Steps 1 and 2 are done by the caller to produce `m_hash`.

            // Step 3 is done by `PSSMetrics::new()` above.

            // Step 5, out of order.
            let masked_db = em.skip_and_get_input(metrics.db_len)?;
            let h_hash = em.skip_and_get_input(metrics.h_len)?;

            // Step 4.
            if em.read_byte()? != 0xbc {
                return Err(error::Unspecified);
            }

            // Step 7.
            let mut db = [0u8; PUBLIC_KEY_PUBLIC_MODULUS_MAX_LEN];
            let db = &mut db[..metrics.db_len];

            mgf1(self.digest_alg, h_hash.as_slice_less_safe(), db)?;

            masked_db.read_all(error::Unspecified, |masked_bytes| {
                // Step 6. Check the top bits of first byte are zero.
                let b = masked_bytes.read_byte()?;
                if b & !metrics.top_byte_mask != 0 {
                    return Err(error::Unspecified);
                }
                db[0] ^= b;

                // Step 8.
                for i in 1..db.len() {
                    db[i] ^= masked_bytes.read_byte()?;
                }
                Ok(())
            })?;

            // Step 9.
            db[0] &= metrics.top_byte_mask;

            // Step 10.
            let ps_len = metrics.ps_len;
            for i in 0..ps_len {
                if db[i] != 0 {
                    return Err(error::Unspecified);
                }
            }
            if db[metrics.ps_len] != 1 {
                return Err(error::Unspecified);
            }

            // Step 11.
            let salt = &db[(db.len() - metrics.s_len)..];

            // Step 12 and 13.
            let h_prime = pss_digest(self.digest_alg, m_hash, salt);

            // Step 14.
            if h_hash != h_prime.as_ref() {
                return Err(error::Unspecified);
            }

            Ok(())
        }
    }

    struct PSSMetrics {
        #[cfg_attr(not(feature = "rsa_signing"), allow(dead_code))] em_len: usize,
        db_len: usize,
        ps_len: usize,
        s_len: usize,
        h_len: usize,
        top_byte_mask: u8,
    }

    impl PSSMetrics {
        fn new(digest_alg: &'static digest::Algorithm, mod_bits: bits::BitLength)
               -> Result<PSSMetrics, error::Unspecified> {
            let em_bits = mod_bits.try_sub(bits::ONE)?;
            let em_len = em_bits.as_usize_bytes_rounded_up();
            let leading_zero_bits = (8 * em_len) - em_bits.as_usize_bits();
            debug_assert!(leading_zero_bits < 8);
            let top_byte_mask = 0xffu8 >> leading_zero_bits;

            let h_len = digest_alg.output_len;

            // We require the salt length to be equal to the digest length.
            let s_len = h_len;

            // Step 3 of both `EMSA-PSS-ENCODE` is `EMSA-PSS-VERIFY` requires that
            // we reject inputs where "emLen < hLen + sLen + 2". The definition of
            // `emBits` in RFC 3447 Sections 9.1.1 and 9.1.2 says `emBits` must be
            // "at least 8hLen + 8sLen + 9". Since 9 bits requires two bytes, these
            // two conditions are equivalent. 9 bits are required as the 0x01
            // before the salt requires 1 bit and the 0xbc after the digest
            // requires 8 bits.
            let db_len = em_len.checked_sub(1 + s_len).ok_or(error::Unspecified)?;
            let ps_len = db_len.checked_sub(h_len + 1).ok_or(error::Unspecified)?;

            debug_assert!(em_bits.as_usize_bits() >= (8 * h_len) + (8 * s_len) + 9);

            Ok(PSSMetrics {
                em_len: em_len,
                db_len: db_len,
                ps_len: ps_len,
                s_len: s_len,
                h_len: h_len,
                top_byte_mask: top_byte_mask,
            })
        }
    }

    // Mask-generating function MGF1 as described in
    // https://tools.ietf.org/html/rfc3447#appendix-B.2.1.
    fn mgf1(digest_alg: &'static digest::Algorithm, seed: &[u8], mask: &mut [u8])
            -> Result<(), error::Unspecified> {
        let digest_len = digest_alg.output_len;

        // Maximum counter value is the value of (mask_len / digest_len) rounded up.
        let ctr_max = (mask.len() - 1) / digest_len;
        assert!(ctr_max <= u32::max_value() as usize);
        for (i, mask_chunk) in mask.chunks_mut(digest_len).enumerate() {
            let mut ctx = digest::Context::new(digest_alg);
            ctx.update(seed);
            ctx.update(&polyfill::slice::be_u8_from_u32(i as u32));
            let digest = ctx.finish();
            let mask_chunk_len = mask_chunk.len();
            mask_chunk.copy_from_slice(&digest.as_ref()[..mask_chunk_len]);
        }

        Ok(())
    }

    fn pss_digest(digest_alg: &'static digest::Algorithm, m_hash: &digest::Digest,
                  salt: &[u8]) -> digest::Digest {
        // Fixed prefix.
        const PREFIX_ZEROS: [u8; 8] = [0u8; 8];

        // Encoding step 5 and 6, Verification step 12 and 13.
        let mut ctx = digest::Context::new(digest_alg);
        ctx.update(&PREFIX_ZEROS);
        ctx.update(m_hash.as_ref());
        ctx.update(salt);
        ctx.finish()
    }

    macro_rules! rsa_pss_padding {
        ( $PADDING_ALGORITHM:ident, $digest_alg:expr, $doc_str:expr ) => {
            #[doc=$doc_str]
            /// Feature: `rsa_signing`.
            pub static $PADDING_ALGORITHM: PSS = PSS {
                digest_alg: $digest_alg,
            };
        }
    }

    rsa_pss_padding!(RSA_PSS_SHA256, &digest::SHA256,
                     "RSA PSS padding using SHA-256 for RSA signatures.\n\nSee
                     \"`RSA_PSS_*` Details\" in `ring::signature`'s module-level
                     documentation for more details.");
    rsa_pss_padding!(RSA_PSS_SHA384, &digest::SHA384,
                     "RSA PSS padding using SHA-384 for RSA signatures.\n\nSee
                     \"`RSA_PSS_*` Details\" in `ring::signature`'s module-level
                     documentation for more details.");
    rsa_pss_padding!(RSA_PSS_SHA512, &digest::SHA512,
                     "RSA PSS padding using SHA-512 for RSA signatures.\n\nSee
                     \"`RSA_PSS_*` Details\" in `ring::signature`'s module-level
                     documentation for more details.");

    #[cfg(test)]
    mod test {
        use {digest, error, test};
        use super::*;
        use untrusted;

        #[test]
        fn test_pss_padding_verify() {
            test::from_file("src/rsa/rsa_pss_padding_tests.txt",
                            |section, test_case| {
                assert_eq!(section, "");

                let digest_name = test_case.consume_string("Digest");
                let alg = match digest_name.as_ref() {
                    "SHA256" => &RSA_PSS_SHA256,
                    "SHA384" => &RSA_PSS_SHA384,
                    "SHA512" => &RSA_PSS_SHA512,
                    _ =>  { panic!("Unsupported digest: {}", digest_name) }
                };

                let msg = test_case.consume_bytes("Msg");
                let msg = untrusted::Input::from(&msg);
                let m_hash = digest::digest(alg.digest_alg(),
                                            msg.as_slice_less_safe());

                let encoded = test_case.consume_bytes("EM");
                let encoded = untrusted::Input::from(&encoded);

                // Salt is recomputed in verification algorithm.
                let _ = test_case.consume_bytes("Salt");

                let bit_len = test_case.consume_usize_bits("Len");
                let expected_result = test_case.consume_string("Result");

                let actual_result =
                    encoded.read_all(error::Unspecified,
                                     |m| alg.verify(&m_hash, m, bit_len));
                assert_eq!(actual_result.is_ok(), expected_result == "P");

                Ok(())
            });
        }

        // Tests PSS encoding for various public modulus lengths.
        #[cfg(feature = "rsa_signing")]
        #[test]
        fn test_pss_padding_encode() {
            test::from_file("src/rsa/rsa_pss_padding_tests.txt",
                            |section, test_case| {
                assert_eq!(section, "");

                let digest_name = test_case.consume_string("Digest");
                let alg = match digest_name.as_ref() {
                    "SHA256" => &RSA_PSS_SHA256,
                    "SHA384" => &RSA_PSS_SHA384,
                    "SHA512" => &RSA_PSS_SHA512,
                    _ =>  { panic!("Unsupported digest: {}", digest_name) }
                };

                let msg = test_case.consume_bytes("Msg");
                let salt = test_case.consume_bytes("Salt");
                let encoded = test_case.consume_bytes("EM");
                let bit_len = test_case.consume_usize_bits("Len");
                let expected_result = test_case.consume_string("Result");

                // Only test the valid outputs
                if expected_result != "P" {
                    return Ok(())
                }

                let rng = test::rand::FixedSliceRandom { bytes: &salt };

                let mut m_out = vec![0u8; bit_len.as_usize_bytes_rounded_up()];
                let digest = digest::digest(alg.digest_alg(), &msg);
                alg.encode(&digest, &mut m_out, bit_len, &rng).unwrap();
                assert_eq!(m_out, encoded);

                Ok(())
            });
        }
    }

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