Implement --identity command

src/p256.rs

@@ -1,10 +1,14 @@
 use bech32::ToBase32;
 use elliptic_curve::sec1::EncodedPoint;
 use p256::NistP256;
+use sha2::{Digest, Sha256};
+use std::convert::TryInto;
 use std::fmt;
 
 use crate::RECIPIENT_PREFIX;
 
+pub(crate) const TAG_BYTES: usize = 4;
+
 /// Wrapper around a compressed secp256r1 curve point.
 #[derive(Clone)]
 pub struct Recipient(EncodedPoint<NistP256>);
@@ -43,4 +47,9 @@ impl Recipient {
     pub(crate) fn as_bytes(&self) -> &[u8] {
         self.0.as_bytes()
     }
+
+    pub(crate) fn tag(&self) -> [u8; TAG_BYTES] {
+        let tag = Sha256::digest(self.to_string().as_bytes());
+        (&tag[0..TAG_BYTES]).try_into().expect("length is correct")
+    }
 }