Use SHA256 for SSH signing

This also removes (again) support for SHA1.

When using the SSH Agent signers we need to use a wrapper to actually
allow signing with SHA256.

algorithms.go

@@ -5,7 +5,6 @@ import (
 	"crypto/ecdsa"
 	"crypto/hmac"
 	"crypto/rsa"
-	"crypto/sha1"
 	"crypto/sha256"
 	"crypto/sha512"
 	"crypto/subtle" // Use should trigger great care
@@ -68,10 +67,9 @@ var hashToDef = map[crypto.Hash]struct {
 	// http://www.iana.org/assignments/signature-algorithms
 	//
 	// Note that the forbidden hashes have an invalid 'new' function.
-	crypto.MD4: {md4String, func(key []byte) (hash.Hash, error) { return nil, nil }},
-	crypto.MD5: {md5String, func(key []byte) (hash.Hash, error) { return nil, nil }},
-	// Temporarily enable SHA1 because of issue https://github.com/golang/go/issues/37278
-	crypto.SHA1:        {sha1String, func(key []byte) (hash.Hash, error) { return sha1.New(), nil }},
+	crypto.MD4:         {md4String, func(key []byte) (hash.Hash, error) { return nil, nil }},
+	crypto.MD5:         {md5String, func(key []byte) (hash.Hash, error) { return nil, nil }},
+	crypto.SHA1:        {sha1String, func(key []byte) (hash.Hash, error) { return nil, nil }},
 	crypto.SHA224:      {sha224String, func(key []byte) (hash.Hash, error) { return sha256.New224(), nil }},
 	crypto.SHA256:      {sha256String, func(key []byte) (hash.Hash, error) { return sha256.New(), nil }},
 	crypto.SHA384:      {sha384String, func(key []byte) (hash.Hash, error) { return sha512.New384(), nil }},
@@ -192,11 +190,49 @@ func (r *rsaAlgorithm) setSig(b []byte) error {
 	return nil
 }
 
+// Code from https://github.com/cloudtools/ssh-cert-authority/pull/49/files
+// This interface provides a way to reach the exported, but not accessible SignWithOpts() method
+// in x/crypto/ssh/agent. Access to this is needed to sign with more secure signing algorithms
+type agentKeyringSigner interface {
+	SignWithOpts(rand io.Reader, data []byte, opts crypto.SignerOpts) (*ssh.Signature, error)
+}
+
+// A struct to wrap an SSH Signer with one that will switch to SHA256 Signatures.
+// Replaces the call to Sign() with a call to SignWithOpts using HashFunc() algorithm.
+type Sha256Signer struct {
+	ssh.Signer
+}
+
+func (s Sha256Signer) HashFunc() crypto.Hash {
+	return crypto.SHA256
+}
+
+func (s Sha256Signer) Sign(rand io.Reader, data []byte) (*ssh.Signature, error) {
+	if aks, ok := s.Signer.(agentKeyringSigner); !ok {
+		return nil, fmt.Errorf("ssh: can't wrap a non ssh agentKeyringSigner")
+	} else {
+		return aks.SignWithOpts(rand, data, s)
+	}
+}
+
 func (r *rsaAlgorithm) Sign(rand io.Reader, p crypto.PrivateKey, sig []byte) ([]byte, error) {
 	if r.sshSigner != nil {
-		sshsig, err := r.sshSigner.Sign(rand, sig)
-		if err != nil {
-			return nil, err
+		var (
+			sshsig *ssh.Signature
+			err    error
+		)
+		// are we using an SSH Agent
+		if _, ok := r.sshSigner.(agentKeyringSigner); ok {
+			signer := Sha256Signer{r.sshSigner}
+			sshsig, err = signer.Sign(rand, sig)
+			if err != nil {
+				return nil, err
+			}
+		} else {
+			sshsig, err = r.sshSigner.(ssh.AlgorithmSigner).SignWithAlgorithm(rand, sig, ssh.SigAlgoRSASHA2256)
+			if err != nil {
+				return nil, err
+			}
 		}
 
 		return sshsig.Blob, nil

httpsig.go

@@ -43,7 +43,6 @@ const (
 	BLAKE2B_384      Algorithm = blake2b_384String
 	BLAKE2B_512      Algorithm = blake2b_512String
 	// RSA-based algorithms.
-	RSA_SHA1   Algorithm = rsaPrefix + "-" + sha1String
 	RSA_SHA224 Algorithm = rsaPrefix + "-" + sha224String
 	// RSA_SHA256 is the default algorithm.
 	RSA_SHA256    Algorithm = rsaPrefix + "-" + sha256String
@@ -234,7 +233,7 @@ func getSSHAlgorithm(pkType string) Algorithm {
 	case strings.HasPrefix(pkType, sshPrefix+"-"+ed25519Prefix):
 		return ED25519
 	case strings.HasPrefix(pkType, sshPrefix+"-"+rsaPrefix):
-		return RSA_SHA1
+		return RSA_SHA256
 	}
 
 	return ""
@@ -324,7 +323,6 @@ func newSSHSigner(sshSigner ssh.Signer, algo Algorithm, dAlgo DigestAlgorithm, h
 }
 
 func newSigner(algo Algorithm, dAlgo DigestAlgorithm, headers []string, scheme SignatureScheme, expiresIn int64) (Signer, error) {
-
 	var expires, created int64 = 0, 0
 	if expiresIn != 0 {
 		created = time.Now().Unix()