io.jwt.decode (#140)

* io.jwt.decode

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

* Update README

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

* Install musl-tools to compile ring crate

ring crate is a dependency of jsonwebtoken

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

---------

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

src/builtins/jwt.rs

@@ -3,19 +3,64 @@
 
 use crate::ast::{Expr, Ref};
 use crate::builtins;
-use crate::builtins::utils::ensure_args_count;
+use crate::builtins::utils::{ensure_args_count, ensure_string};
 
 use crate::lexer::Span;
 use crate::value::Value;
 
+use itertools::Itertools;
 use std::collections::HashMap;
 
-use anyhow::Result;
+use anyhow::{bail, Result};
 
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
+    m.insert("io.jwt.decode", (jwt_decode, 1));
     m.insert("io.jwt.decode_verify", (jwt_decode_verify, 2));
 }
 
+fn decode(span: &Span, jwt: String, strict: bool) -> Result<Value> {
+    let Some((Ok(header), Ok(payload), Ok(signature))) = jwt
+        .split('.')
+        .map(|p| data_encoding::BASE64URL_NOPAD.decode(p.as_bytes()))
+        .collect_tuple()
+    else {
+        if strict {
+            bail!(span.error("invalid jwt token"));
+        }
+        return Ok(Value::Undefined);
+    };
+
+    let header = String::from_utf8_lossy(&header).to_string();
+    let payload = String::from_utf8_lossy(&payload).to_string();
+    let signature = data_encoding::HEXLOWER_PERMISSIVE.encode(&signature);
+
+    let signature = Value::String(signature.into());
+    let header = Value::from_json_str(&header)?;
+
+    if header["enc"] != Value::Undefined {
+        bail!(span.error("JWT is a JWE object, which is not supported"));
+    }
+
+    if header["cty"] == "JWT".into() {
+        if payload.len() <= 2 || !payload.starts_with('"') || !payload.ends_with('"') {
+            bail!(span.error("invalid nested JWT"));
+        }
+        // Ignore ""
+        decode(span, payload[1..payload.len() - 1].to_string(), strict)
+    } else {
+        let payload = Value::from_json_str(&payload)?;
+        Ok(Value::from_array([header, payload, signature].into()))
+    }
+}
+
+fn jwt_decode(span: &Span, params: &[Ref<Expr>], args: &[Value], strict: bool) -> Result<Value> {
+    let name = "io.jwt.decode";
+    ensure_args_count(span, name, params, args, 1)?;
+    let jwt = ensure_string(name, &params[0], &args[0])?;
+
+    decode(span, jwt.to_string(), strict) //header, payload, signature, strict)
+}
+
 fn jwt_decode_verify(
     span: &Span,
     params: &[Ref<Expr>],