1 files changed, 71 insertions, 0 deletions
diff --git a/tools/genpem.go b/tools/genpem.go
new file mode 100644
index 0000000..542fd9a
--- /dev/null
+++ b/tools/genpem.go
@@ -0,0 +1,71 @@
+package main
+
+import (
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"math/big"
+	"os"
+)
+
+const (
+	P = "1201758001370723323398753778257470257713354828752713123415294815" +
+		"0506251412291888866940292054989907714155267326586216043845592229" +
+		"084368540020196135619327879"
+
+	Q = "1189892136861686835188050824611210139447876026576932541274639840" +
+		"5473436969889506919017477758618276066588858607419440134394668095" +
+		"105156501566867770737187273"
+
+	E = "65537"
+)
+
+func main() {
+	// NOTE(fusion): Generate key from known P, Q, and E. There isn't a helper
+	// function from the standard library so we need to build the private key
+	// ourselves.
+	p, ok := new(big.Int).SetString(P, 10)
+	if !ok || !p.ProbablyPrime(4) {
+		panic("invalid P")
+	}
+
+	q, ok := new(big.Int).SetString(Q, 10)
+	if !ok || !q.ProbablyPrime(4) {
+		panic("invalid Q")
+	}
+
+	e, ok := new(big.Int).SetString(E, 10)
+	if !ok || !e.IsInt64() || !e.ProbablyPrime(4) {
+		panic("invalid E")
+	}
+
+	pMinus1 := new(big.Int).Sub(p, big.NewInt(1))
+	qMinus1 := new(big.Int).Sub(q, big.NewInt(1))
+	gcd := new(big.Int).GCD(nil, nil, pMinus1, qMinus1)
+	phi := new(big.Int).Mul(pMinus1, qMinus1)
+	lambda := new(big.Int).Div(phi, gcd)
+	d := new(big.Int).ModInverse(e, lambda)
+	n := new(big.Int).Mul(p, q)
+
+	privateKey := rsa.PrivateKey{
+		PublicKey: rsa.PublicKey{
+			N: n,
+			E: int(e.Int64()),
+		},
+		D:      d,
+		Primes: []*big.Int{p, q},
+	}
+
+	// NOTE(fusion): `Validate()` will only perform minor sanity checks. To actually
+	// check that the output key is valid use `openssl rsa -in KEY.PEM -check`.
+	if err := privateKey.Validate(); err != nil {
+		panic("invalid private key: " + err.Error())
+	}
+
+	// NOTE(fusion): Dump private key into stdout.
+	block := pem.Block{
+		Type:  "RSA PRIVATE KEY",
+		Bytes: x509.MarshalPKCS1PrivateKey(&privateKey),
+	}
+	pem.Encode(os.Stdout, &block)
+}