add ecdsa page

malta.config.json

@@ -20,6 +20,12 @@
 				["Open redirect", "/open-redirect"]
 			]
 		},
+		{
+			"title": "Cryptography",
+			"pages": [
+				["ECDSA", "/cryptography/ecdsa"]
+			]
+		},
 		{
 			"title": "Links",
 			"pages": [

pages/cryptography/ecdsa.md

@@ -0,0 +1,83 @@
+---
+title: "Elliptic curve digital signature algorithm (ECDSA)"
+---
+
+# Elliptic curve digital signature algorithm (ECDSA)
+
+ECDSA is a digital signature algorithm using elliptic-curve cryptography. A private key is used to sign a message and a public key is used verify the signature.
+
+The message is hashed with algorithms like SHA-256 before signing.
+
+```go
+import (
+	"crypto/ecdsa"
+	"crypto/rand"
+	"crypto/sha256"
+)
+
+msg := "Hello world!"
+hash := sha256.Sum256([]byte(msg))
+signature, err := ecdsa.SignASN1(rand.Reader, privateKey, hash[:])
+```
+
+## Signatures
+
+ECDSA signatures are represented using a pair of positive integers, (r, s).
+
+### IEEE P1363
+
+In the IEEE P1363 format, the signature is the concatenation of r and s. The values are encoded as big-endian bytes with a size equivalent to the curve size. For example, P-256 is 256 bits or 32 bytes in size.
+
+```ts
+r || s;
+```
+
+### PKIX
+
+In [RFC 5480](https://datatracker.ietf.org/doc/html/rfc5480) by the PKIX working group, the signature is ASN.1 DER encoded sequence of r and s.
+
+```
+SEQUENCE {
+    r     INTEGER,
+    s     INTEGER
+}
+```
+
+## Public keys
+
+ECDSA public keys are represented as a pair of positive integers, (x, y).
+
+### SEC1
+
+In SEC 1, which defines ECDSA, public keys can either be uncompressed or compressed. Uncompressed keys are the concatenation of x and y, with a leading `0x04` byte. The values are encoded as big-endian bytes with a size equivalent to the curve size. For example, P-256 is 256 bits or 32 bytes in size.
+
+```
+0x04 || x || y
+```
+
+Compressed keys are the x value with a leading `0x02` if x is even or `0x03` byte if odd. The y value can be calculated from x and the curve.
+
+```
+0x02 || x
+0x03 || x
+```
+
+### PKIX
+
+In [RFC 5480](https://datatracker.ietf.org/doc/html/rfc5480) by the PKIX working group, the public key is represented as a `SubjectPublicKeyInfo` ASN.1 sequence. The `subjectPublicKey` is either the compressed or uncompressed SEC1 public key.
+
+```
+SubjectPublicKeyInfo := SEQUENCE {
+    algorithm           AlgorithmIdentifier,
+    subjectPublicKey    BIT STRING
+}
+```
+
+The `AlgorithmIdentifier` for ECDSA is an ASN.1 sequence with the ECDSA object identifier (`1.2.840.10045.2.1`) and the curve (e.g. `1.2.840.10045.3.1.7` for P-256 curve)
+
+```
+AlgorithmIdentifier := SEQUENCE {
+    algorithm   OBJECT IDENTIFIER
+    namedCurve  OBJECT IDENTIFIER
+}
+```