87 lines
2.8 KiB
Go
87 lines
2.8 KiB
Go
/*-
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* Copyright 2014 Square Inc.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package josecipher
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import (
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"bytes"
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"crypto"
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"crypto/ecdsa"
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"crypto/elliptic"
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"encoding/binary"
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)
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// DeriveECDHES derives a shared encryption key using ECDH/ConcatKDF as described in JWE/JWA.
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// It is an error to call this function with a private/public key that are not on the same
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// curve. Callers must ensure that the keys are valid before calling this function. Output
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// size may be at most 1<<16 bytes (64 KiB).
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func DeriveECDHES(alg string, apuData, apvData []byte, priv *ecdsa.PrivateKey, pub *ecdsa.PublicKey, size int) []byte {
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if size > 1<<16 {
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panic("ECDH-ES output size too large, must be less than or equal to 1<<16")
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}
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// algId, partyUInfo, partyVInfo inputs must be prefixed with the length
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algID := lengthPrefixed([]byte(alg))
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ptyUInfo := lengthPrefixed(apuData)
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ptyVInfo := lengthPrefixed(apvData)
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// suppPubInfo is the encoded length of the output size in bits
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supPubInfo := make([]byte, 4)
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binary.BigEndian.PutUint32(supPubInfo, uint32(size)*8)
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if !priv.PublicKey.Curve.IsOnCurve(pub.X, pub.Y) {
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panic("public key not on same curve as private key")
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}
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z, _ := priv.Curve.ScalarMult(pub.X, pub.Y, priv.D.Bytes())
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zBytes := z.Bytes()
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// Note that calling z.Bytes() on a big.Int may strip leading zero bytes from
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// the returned byte array. This can lead to a problem where zBytes will be
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// shorter than expected which breaks the key derivation. Therefore we must pad
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// to the full length of the expected coordinate here before calling the KDF.
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octSize := dSize(priv.Curve)
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if len(zBytes) != octSize {
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zBytes = append(bytes.Repeat([]byte{0}, octSize-len(zBytes)), zBytes...)
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}
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reader := NewConcatKDF(crypto.SHA256, zBytes, algID, ptyUInfo, ptyVInfo, supPubInfo, []byte{})
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key := make([]byte, size)
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// Read on the KDF will never fail
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_, _ = reader.Read(key)
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return key
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}
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// dSize returns the size in octets for a coordinate on a elliptic curve.
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func dSize(curve elliptic.Curve) int {
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order := curve.Params().P
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bitLen := order.BitLen()
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size := bitLen / 8
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if bitLen%8 != 0 {
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size++
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}
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return size
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}
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func lengthPrefixed(data []byte) []byte {
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out := make([]byte, len(data)+4)
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binary.BigEndian.PutUint32(out, uint32(len(data)))
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copy(out[4:], data)
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return out
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}
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