100 lines
2.5 KiB
Go
100 lines
2.5 KiB
Go
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//+build !go1.15
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package jwt
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import (
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"crypto/ecdsa"
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"crypto/rand"
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"math/big"
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"math/bits"
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)
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// Implements the Sign method from SigningMethod
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// For this signing method, key must be an ecdsa.PrivateKey struct
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func (m *SigningMethodECDSA) Sign(signingString string, key interface{}) (string, error) {
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// Get the key
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var ecdsaKey *ecdsa.PrivateKey
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switch k := key.(type) {
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case *ecdsa.PrivateKey:
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ecdsaKey = k
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default:
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return "", ErrInvalidKeyType
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}
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// Create the hasher
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if !m.Hash.Available() {
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return "", ErrHashUnavailable
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}
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hasher := m.Hash.New()
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hasher.Write([]byte(signingString))
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// Sign the string and return r, s
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if r, s, err := ecdsa.Sign(rand.Reader, ecdsaKey, hasher.Sum(nil)); err == nil {
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curveBits := ecdsaKey.Curve.Params().BitSize
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if m.CurveBits != curveBits {
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return "", ErrInvalidKey
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}
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keyBytes := curveBits / 8
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if curveBits%8 > 0 {
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keyBytes += 1
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}
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// We serialize the outputs (r and s) into big-endian byte arrays
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// padded with zeros on the left to make sure the sizes work out.
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// Output must be 2*keyBytes long.
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out := make([]byte, 2*keyBytes)
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fillBytesInt(r, out[0:keyBytes]) // r is assigned to the first half of output.
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fillBytesInt(s, out[keyBytes:]) // s is assigned to the second half of output.
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return EncodeSegment(out), nil
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} else {
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return "", err
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}
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}
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func fillBytesInt(x *big.Int, buf []byte) []byte {
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// Clear whole buffer. (This gets optimized into a memclr.)
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for i := range buf {
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buf[i] = 0
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}
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// This code is deeply inspired by go's own implementation but rewritten.
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// Although this function is called bits it returns words
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words := x.Bits()
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// Words are uints as per the definition of bits.Word and thus there are usually (64) /8 bytes per word
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bytesPerWord := bits.UintSize / 8
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// If our buffer is longer than the expected number of words start mid-way
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pos := len(buf) - len(words)*bytesPerWord
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// Now iterate across the words (backwards)
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for i := range words {
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// Grab the last word (Which is the biggest number)
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word := words[len(words)-1-i]
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// Now for each byte in the word
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// [abcd...] we want buf[0] = a, buf[1] = b ...
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for j := bytesPerWord; j > 0; j-- {
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d := byte(word)
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// if our position is less than 0 then panic
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if pos+j-1 >= 0 {
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// set the value of the byte to the byte
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buf[pos+j-1] = d
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} else if d != 0 {
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panic("math/big: buffer too small to fit value") // have to use the same panic string for complete compatibility
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}
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// shift the word 8 bits and reloop.
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word >>= 8
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}
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pos += bytesPerWord
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}
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return buf
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}
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