From 462ddce5b2a47835d057e3950ee5feb9d6c7bc10 Mon Sep 17 00:00:00 2001 From: Luke Champine Date: Fri, 3 Apr 2020 05:57:24 -0400 Subject: [PATCH] crypto/ecies: improve concatKDF (#20836) This removes a bunch of weird code around the counter overflow check in concatKDF and makes it actually work for different hash output sizes. The overflow check worked as follows: concatKDF applies the hash function N times, where N is roundup(kdLen, hashsize) / hashsize. N should not overflow 32 bits because that would lead to a repetition in the KDF output. A couple issues with the overflow check: - It used the hash.BlockSize, which is wrong because the block size is about the input of the hash function. Luckily, all standard hash functions have a block size that's greater than the output size, so concatKDF didn't crash, it just generated too much key material. - The check used big.Int to compare against 2^32-1. - The calculation could still overflow before reaching the check. The new code in concatKDF doesn't check for overflow. Instead, there is a new check on ECIESParams which ensures that params.KeyLen is < 512. This removes any possibility of overflow. There are a couple of miscellaneous improvements bundled in with this change: - The key buffer is pre-allocated instead of appending the hash output to an initially empty slice. - The code that uses concatKDF to derive keys is now shared between Encrypt and Decrypt. - There was a redundant invocation of IsOnCurve in Decrypt. This is now removed because elliptic.Unmarshal already checks whether the input is a valid curve point since Go 1.5. Co-authored-by: Felix Lange --- crypto/ecies/ecies.go | 143 ++++++++++++------------------------- crypto/ecies/ecies_test.go | 40 +++++++---- crypto/ecies/params.go | 19 +++++ 3 files changed, 93 insertions(+), 109 deletions(-) diff --git a/crypto/ecies/ecies.go b/crypto/ecies/ecies.go index 147418148..64b5a99d0 100644 --- a/crypto/ecies/ecies.go +++ b/crypto/ecies/ecies.go @@ -35,6 +35,7 @@ import ( "crypto/elliptic" "crypto/hmac" "crypto/subtle" + "encoding/binary" "fmt" "hash" "io" @@ -44,7 +45,6 @@ import ( var ( ErrImport = fmt.Errorf("ecies: failed to import key") ErrInvalidCurve = fmt.Errorf("ecies: invalid elliptic curve") - ErrInvalidParams = fmt.Errorf("ecies: invalid ECIES parameters") ErrInvalidPublicKey = fmt.Errorf("ecies: invalid public key") ErrSharedKeyIsPointAtInfinity = fmt.Errorf("ecies: shared key is point at infinity") ErrSharedKeyTooBig = fmt.Errorf("ecies: shared key params are too big") @@ -138,57 +138,39 @@ func (prv *PrivateKey) GenerateShared(pub *PublicKey, skLen, macLen int) (sk []b } var ( - ErrKeyDataTooLong = fmt.Errorf("ecies: can't supply requested key data") ErrSharedTooLong = fmt.Errorf("ecies: shared secret is too long") ErrInvalidMessage = fmt.Errorf("ecies: invalid message") ) -var ( - big2To32 = new(big.Int).Exp(big.NewInt(2), big.NewInt(32), nil) - big2To32M1 = new(big.Int).Sub(big2To32, big.NewInt(1)) -) - -func incCounter(ctr []byte) { - if ctr[3]++; ctr[3] != 0 { - return - } - if ctr[2]++; ctr[2] != 0 { - return - } - if ctr[1]++; ctr[1] != 0 { - return - } - if ctr[0]++; ctr[0] != 0 { - return - } -} - // NIST SP 800-56 Concatenation Key Derivation Function (see section 5.8.1). -func concatKDF(hash hash.Hash, z, s1 []byte, kdLen int) (k []byte, err error) { - if s1 == nil { - s1 = make([]byte, 0) - } - - reps := ((kdLen + 7) * 8) / (hash.BlockSize() * 8) - if big.NewInt(int64(reps)).Cmp(big2To32M1) > 0 { - fmt.Println(big2To32M1) - return nil, ErrKeyDataTooLong - } - - counter := []byte{0, 0, 0, 1} - k = make([]byte, 0) - - for i := 0; i <= reps; i++ { - hash.Write(counter) +func concatKDF(hash hash.Hash, z, s1 []byte, kdLen int) []byte { + counterBytes := make([]byte, 4) + k := make([]byte, 0, roundup(kdLen, hash.Size())) + for counter := uint32(1); len(k) < kdLen; counter++ { + binary.BigEndian.PutUint32(counterBytes, counter) + hash.Reset() + hash.Write(counterBytes) hash.Write(z) hash.Write(s1) - k = append(k, hash.Sum(nil)...) - hash.Reset() - incCounter(counter) + k = hash.Sum(k) } + return k[:kdLen] +} - k = k[:kdLen] - return +// roundup rounds size up to the next multiple of blocksize. +func roundup(size, blocksize int) int { + return size + blocksize - (size % blocksize) +} + +// deriveKeys creates the encryption and MAC keys using concatKDF. +func deriveKeys(hash hash.Hash, z, s1 []byte, keyLen int) (Ke, Km []byte) { + K := concatKDF(hash, z, s1, 2*keyLen) + Ke = K[:keyLen] + Km = K[keyLen:] + hash.Reset() + hash.Write(Km) + Km = hash.Sum(Km[:0]) + return Ke, Km } // messageTag computes the MAC of a message (called the tag) as per @@ -209,7 +191,6 @@ func generateIV(params *ECIESParams, rand io.Reader) (iv []byte, err error) { } // symEncrypt carries out CTR encryption using the block cipher specified in the -// parameters. func symEncrypt(rand io.Reader, params *ECIESParams, key, m []byte) (ct []byte, err error) { c, err := params.Cipher(key) if err != nil { @@ -249,36 +230,27 @@ func symDecrypt(params *ECIESParams, key, ct []byte) (m []byte, err error) { // ciphertext. s1 is fed into key derivation, s2 is fed into the MAC. If the // shared information parameters aren't being used, they should be nil. func Encrypt(rand io.Reader, pub *PublicKey, m, s1, s2 []byte) (ct []byte, err error) { - params := pub.Params - if params == nil { - if params = ParamsFromCurve(pub.Curve); params == nil { - err = ErrUnsupportedECIESParameters - return - } + params, err := pubkeyParams(pub) + if err != nil { + return nil, err } + R, err := GenerateKey(rand, pub.Curve, params) if err != nil { - return + return nil, err } - hash := params.Hash() z, err := R.GenerateShared(pub, params.KeyLen, params.KeyLen) if err != nil { - return - } - K, err := concatKDF(hash, z, s1, params.KeyLen+params.KeyLen) - if err != nil { - return + return nil, err } - Ke := K[:params.KeyLen] - Km := K[params.KeyLen:] - hash.Write(Km) - Km = hash.Sum(nil) - hash.Reset() + + hash := params.Hash() + Ke, Km := deriveKeys(hash, z, s1, params.KeyLen) em, err := symEncrypt(rand, params, Ke, m) if err != nil || len(em) <= params.BlockSize { - return + return nil, err } d := messageTag(params.Hash, Km, em, s2) @@ -288,7 +260,7 @@ func Encrypt(rand io.Reader, pub *PublicKey, m, s1, s2 []byte) (ct []byte, err e copy(ct, Rb) copy(ct[len(Rb):], em) copy(ct[len(Rb)+len(em):], d) - return + return ct, nil } // Decrypt decrypts an ECIES ciphertext. @@ -296,13 +268,11 @@ func (prv *PrivateKey) Decrypt(c, s1, s2 []byte) (m []byte, err error) { if len(c) == 0 { return nil, ErrInvalidMessage } - params := prv.PublicKey.Params - if params == nil { - if params = ParamsFromCurve(prv.PublicKey.Curve); params == nil { - err = ErrUnsupportedECIESParameters - return - } + params, err := pubkeyParams(&prv.PublicKey) + if err != nil { + return nil, err } + hash := params.Hash() var ( @@ -316,12 +286,10 @@ func (prv *PrivateKey) Decrypt(c, s1, s2 []byte) (m []byte, err error) { case 2, 3, 4: rLen = (prv.PublicKey.Curve.Params().BitSize + 7) / 4 if len(c) < (rLen + hLen + 1) { - err = ErrInvalidMessage - return + return nil, ErrInvalidMessage } default: - err = ErrInvalidPublicKey - return + return nil, ErrInvalidPublicKey } mStart = rLen @@ -331,36 +299,19 @@ func (prv *PrivateKey) Decrypt(c, s1, s2 []byte) (m []byte, err error) { R.Curve = prv.PublicKey.Curve R.X, R.Y = elliptic.Unmarshal(R.Curve, c[:rLen]) if R.X == nil { - err = ErrInvalidPublicKey - return - } - if !R.Curve.IsOnCurve(R.X, R.Y) { - err = ErrInvalidCurve - return + return nil, ErrInvalidPublicKey } z, err := prv.GenerateShared(R, params.KeyLen, params.KeyLen) if err != nil { - return + return nil, err } - - K, err := concatKDF(hash, z, s1, params.KeyLen+params.KeyLen) - if err != nil { - return - } - - Ke := K[:params.KeyLen] - Km := K[params.KeyLen:] - hash.Write(Km) - Km = hash.Sum(nil) - hash.Reset() + Ke, Km := deriveKeys(hash, z, s1, params.KeyLen) d := messageTag(params.Hash, Km, c[mStart:mEnd], s2) if subtle.ConstantTimeCompare(c[mEnd:], d) != 1 { - err = ErrInvalidMessage - return + return nil, ErrInvalidMessage } - m, err = symDecrypt(params, Ke, c[mStart:mEnd]) - return + return symDecrypt(params, Ke, c[mStart:mEnd]) } diff --git a/crypto/ecies/ecies_test.go b/crypto/ecies/ecies_test.go index b465f076f..0a6aeb2b5 100644 --- a/crypto/ecies/ecies_test.go +++ b/crypto/ecies/ecies_test.go @@ -42,17 +42,23 @@ import ( "github.com/ethereum/go-ethereum/crypto" ) -// Ensure the KDF generates appropriately sized keys. func TestKDF(t *testing.T) { - msg := []byte("Hello, world") - h := sha256.New() - - k, err := concatKDF(h, msg, nil, 64) - if err != nil { - t.Fatal(err) - } - if len(k) != 64 { - t.Fatalf("KDF: generated key is the wrong size (%d instead of 64\n", len(k)) + tests := []struct { + length int + output []byte + }{ + {6, decode("858b192fa2ed")}, + {32, decode("858b192fa2ed4395e2bf88dd8d5770d67dc284ee539f12da8bceaa45d06ebae0")}, + {48, decode("858b192fa2ed4395e2bf88dd8d5770d67dc284ee539f12da8bceaa45d06ebae0700f1ab918a5f0413b8140f9940d6955")}, + {64, decode("858b192fa2ed4395e2bf88dd8d5770d67dc284ee539f12da8bceaa45d06ebae0700f1ab918a5f0413b8140f9940d6955f3467fd6672cce1024c5b1effccc0f61")}, + } + + for _, test := range tests { + h := sha256.New() + k := concatKDF(h, []byte("input"), nil, test.length) + if !bytes.Equal(k, test.output) { + t.Fatalf("KDF: generated key %x does not match expected output %x", k, test.output) + } } } @@ -293,8 +299,8 @@ func TestParamSelection(t *testing.T) { func testParamSelection(t *testing.T, c testCase) { params := ParamsFromCurve(c.Curve) - if params == nil && c.Expected != nil { - t.Fatalf("%s (%s)\n", ErrInvalidParams.Error(), c.Name) + if params == nil { + t.Fatal("ParamsFromCurve returned nil") } else if params != nil && !cmpParams(params, c.Expected) { t.Fatalf("ecies: parameters should be invalid (%s)\n", c.Name) } @@ -401,7 +407,7 @@ func TestSharedKeyStatic(t *testing.T) { t.Fatal(ErrBadSharedKeys) } - sk, _ := hex.DecodeString("167ccc13ac5e8a26b131c3446030c60fbfac6aa8e31149d0869f93626a4cdf62") + sk := decode("167ccc13ac5e8a26b131c3446030c60fbfac6aa8e31149d0869f93626a4cdf62") if !bytes.Equal(sk1, sk) { t.Fatalf("shared secret mismatch: want: %x have: %x", sk, sk1) } @@ -414,3 +420,11 @@ func hexKey(prv string) *PrivateKey { } return ImportECDSA(key) } + +func decode(s string) []byte { + bytes, err := hex.DecodeString(s) + if err != nil { + panic(err) + } + return bytes +} diff --git a/crypto/ecies/params.go b/crypto/ecies/params.go index 6312daf5a..0bd3877dd 100644 --- a/crypto/ecies/params.go +++ b/crypto/ecies/params.go @@ -49,8 +49,14 @@ var ( DefaultCurve = ethcrypto.S256() ErrUnsupportedECDHAlgorithm = fmt.Errorf("ecies: unsupported ECDH algorithm") ErrUnsupportedECIESParameters = fmt.Errorf("ecies: unsupported ECIES parameters") + ErrInvalidKeyLen = fmt.Errorf("ecies: invalid key size (> %d) in ECIESParams", maxKeyLen) ) +// KeyLen is limited to prevent overflow of the counter +// in concatKDF. While the theoretical limit is much higher, +// no known cipher uses keys larger than 512 bytes. +const maxKeyLen = 512 + type ECIESParams struct { Hash func() hash.Hash // hash function hashAlgo crypto.Hash @@ -115,3 +121,16 @@ func AddParamsForCurve(curve elliptic.Curve, params *ECIESParams) { func ParamsFromCurve(curve elliptic.Curve) (params *ECIESParams) { return paramsFromCurve[curve] } + +func pubkeyParams(key *PublicKey) (*ECIESParams, error) { + params := key.Params + if params == nil { + if params = ParamsFromCurve(key.Curve); params == nil { + return nil, ErrUnsupportedECIESParameters + } + } + if params.KeyLen > maxKeyLen { + return nil, ErrInvalidKeyLen + } + return params, nil +}