Initial structure

internal/crypto/cert.go

@@ -0,0 +1,77 @@
+package crypto
+
+import (
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/sha256"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/hex"
+	"encoding/pem"
+	"errors"
+	"fmt"
+	"math/big"
+	"time"
+)
+
+// CertValidity is how long self-signed certs are valid for. We use a
+// long horizon because cert rotation is operator-driven, not automatic.
+const CertValidity = 10 * 365 * 24 * time.Hour
+
+// buildSelfSignedCert produces an X.509 certificate signed by `priv`
+// itself, using the given common name. Returns the DER bytes.
+func buildSelfSignedCert(priv *rsa.PrivateKey, commonName string) ([]byte, error) {
+	serial, err := rand.Int(rand.Reader, new(big.Int).Lsh(big.NewInt(1), 128))
+	if err != nil {
+		return nil, err
+	}
+	tmpl := &x509.Certificate{
+		SerialNumber:          serial,
+		Subject:               pkix.Name{CommonName: commonName, Organization: []string{"quptime"}},
+		NotBefore:             time.Now().Add(-1 * time.Hour),
+		NotAfter:              time.Now().Add(CertValidity),
+		KeyUsage:              x509.KeyUsageDigitalSignature | x509.KeyUsageKeyEncipherment,
+		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth, x509.ExtKeyUsageClientAuth},
+		BasicConstraintsValid: true,
+		IsCA:                  false,
+	}
+	return x509.CreateCertificate(rand.Reader, tmpl, tmpl, &priv.PublicKey, priv)
+}
+
+// Fingerprint computes the SHA-256 fingerprint of an X.509 certificate's
+// SubjectPublicKeyInfo (the same hash used by `openssl x509 -pubkey -noout
+// | openssl dgst -sha256`). Returns the lowercase hex digest with a
+// "sha256:" prefix to match SSH conventions.
+func Fingerprint(cert *x509.Certificate) string {
+	return FingerprintFromSPKI(cert.RawSubjectPublicKeyInfo)
+}
+
+// FingerprintFromSPKI is the underlying helper.
+func FingerprintFromSPKI(spki []byte) string {
+	sum := sha256.Sum256(spki)
+	return "sha256:" + hex.EncodeToString(sum[:])
+}
+
+// FingerprintFromCertPEM parses a PEM-encoded certificate and returns
+// its fingerprint.
+func FingerprintFromCertPEM(certPEM []byte) (string, error) {
+	block, _ := pem.Decode(certPEM)
+	if block == nil {
+		return "", errors.New("cert: no PEM block")
+	}
+	cert, err := x509.ParseCertificate(block.Bytes)
+	if err != nil {
+		return "", fmt.Errorf("parse cert: %w", err)
+	}
+	return Fingerprint(cert), nil
+}
+
+// FingerprintFromPubKeyPEM parses a public-key PEM and returns its
+// fingerprint over the same SPKI bytes.
+func FingerprintFromPubKeyPEM(pubPEM []byte) (string, error) {
+	block, _ := pem.Decode(pubPEM)
+	if block == nil {
+		return "", errors.New("pubkey: no PEM block")
+	}
+	return FingerprintFromSPKI(block.Bytes), nil
+}