// Copyright 2017 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package trs

import (
	"crypto"
	"crypto/ecdsa"
	"crypto/rsa"
	"encoding/asn1"
	"errors"
	"fmt"
)

// pickSignatureAlgorithm selects a signature algorithm that is compatible with
// the given public key and the list of algorithms from both sides of connection.
// The lists of signature algorithms (peerSigAlgs and ourSigAlgs) are ignored
// for tlsVersion < VersionTLS12.
//
// The returned SignatureScheme codepoint is only meaningful for TLS 1.2 and newer
// previous TLS versions have a fixed hash function.
func pickSignatureAlgorithm(pubkey crypto.PublicKey, peerSigAlgs, ourSigAlgs []SignatureScheme, tlsVersion uint16) (SignatureScheme, uint8, crypto.Hash, error) {
	if tlsVersion < VersionTLS12 || len(peerSigAlgs) == 0 {
		// If the client didn't specify any signature_algorithms
		// extension then we can assume that it supports SHA1. See
		// http://tools.ietf.org/html/rfc5246#section-7.4.1.4.1
		switch pubkey.(type) {
		case *rsa.PublicKey:
			if tlsVersion < VersionTLS12 {
				return 0, signaturePKCS1v15, crypto.MD5SHA1, nil
			} else {
				return PKCS1WithSHA1, signaturePKCS1v15, crypto.SHA1, nil
			}
		case *ecdsa.PublicKey:
			return ECDSAWithSHA1, signatureECDSA, crypto.SHA1, nil
		default:
			return 0, 0, 0, fmt.Errorf("tls: unsupported public key: %T", pubkey)
		}
	}
	for _, sigAlg := range peerSigAlgs {
		if !isSupportedSignatureAlgorithm(sigAlg, ourSigAlgs) {
			continue
		}
		hashAlg, err := lookupTLSHash(sigAlg)
		if err != nil {
			panic("tls: supported signature algorithm has an unknown hash function")
		}
		sigType := signatureFromSignatureScheme(sigAlg)
		if (sigType == signaturePKCS1v15 || hashAlg == crypto.SHA1) && tlsVersion >= VersionTLS13 {
			// TLS 1.3 forbids RSASSA-PKCS1-v1_5 and SHA-1 for
			// handshake messages.
			continue
		}
		switch pubkey.(type) {
		case *rsa.PublicKey:
			if sigType == signaturePKCS1v15 || sigType == signatureRSAPSS {
				return sigAlg, sigType, hashAlg, nil
			}
		case *ecdsa.PublicKey:
			if sigType == signatureECDSA {
				return sigAlg, sigType, hashAlg, nil
			}
		}
	}
	return 0, 0, 0, errors.New("tls: peer doesn't support any common signature algorithms")
}

// verifyHandshakeSignature verifies a signature against pre-hashed handshake
// contents.
func verifyHandshakeSignature(sigType uint8, pubkey crypto.PublicKey, hashFunc crypto.Hash, digest, sig []byte) error {
	switch sigType {
	case signatureECDSA:
		pubKey, ok := pubkey.(*ecdsa.PublicKey)
		if !ok {
			return errors.New("tls: ECDSA signing requires a ECDSA public key")
		}
		ecdsaSig := new(ecdsaSignature)
		if _, err := asn1.Unmarshal(sig, ecdsaSig); err != nil {
			return err
		}
		if ecdsaSig.R.Sign() <= 0 || ecdsaSig.S.Sign() <= 0 {
			return errors.New("tls: ECDSA signature contained zero or negative values")
		}
		if !ecdsa.Verify(pubKey, digest, ecdsaSig.R, ecdsaSig.S) {
			return errors.New("tls: ECDSA verification failure")
		}
	case signaturePKCS1v15:
		pubKey, ok := pubkey.(*rsa.PublicKey)
		if !ok {
			return errors.New("tls: RSA signing requires a RSA public key")
		}
		if err := rsa.VerifyPKCS1v15(pubKey, hashFunc, digest, sig); err != nil {
			return err
		}
	case signatureRSAPSS:
		pubKey, ok := pubkey.(*rsa.PublicKey)
		if !ok {
			return errors.New("tls: RSA signing requires a RSA public key")
		}
		signOpts := &rsa.PSSOptions{SaltLength: rsa.PSSSaltLengthEqualsHash}
		if err := rsa.VerifyPSS(pubKey, hashFunc, digest, sig, signOpts); err != nil {
			return err
		}
	default:
		return errors.New("tls: unknown signature algorithm")
	}
	return nil
}