Zk token sdk clean decryption (#23478)

* zk-token-sdk: add decryption for pod elgamal ciphertexts

* zk-token-sdk: add decryption for pod elgamal ciphertexts

* zk-token-sdk: cargo fmt

* zk-token-sdk: minor update to docs

* zk-token-sdk: minor

* zk-token-sdk: fix bpf build error

* zk-token-sdk: more simplifying discrete log

* zk-token-sdk: fmt

* zk-token-sdk: minor update to doc

zk-token-sdk/src/zk_token_elgamal/decryption.rs

@@ -0,0 +1,35 @@
+#[cfg(not(target_arch = "bpf"))]
+use crate::{
+    encryption::elgamal::{ElGamalCiphertext, ElGamalSecretKey},
+    zk_token_elgamal::pod,
+};
+
+#[cfg(not(target_arch = "bpf"))]
+impl pod::ElGamalCiphertext {
+    pub fn decrypt(self, secret_key: &ElGamalSecretKey) -> Option<u64> {
+        let deserialized_ciphertext: Option<ElGamalCiphertext> = self.try_into().ok();
+        if let Some(ciphertext) = deserialized_ciphertext {
+            ciphertext.decrypt_u32(secret_key)
+        } else {
+            None
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use {super::*, crate::encryption::elgamal::ElGamalKeypair};
+
+    #[test]
+    fn test_pod_decryption() {
+        let keypair = ElGamalKeypair::new_rand();
+
+        let pod_ciphertext = pod::ElGamalCiphertext([0u8; 64]);
+        assert_eq!(pod_ciphertext.decrypt(&keypair.secret).unwrap(), 0);
+
+        let amount = 55_u64;
+        let ciphertext = keypair.public.encrypt(amount);
+        let pod_ciphertext: pod::ElGamalCiphertext = ciphertext.into();
+        assert_eq!(pod_ciphertext.decrypt(&keypair.secret).unwrap(), 55);
+    }
+}