diff --git a/coordinator/src/cli.rs b/coordinator/src/cli.rs
index 28897a60..e75e3281 100644
--- a/coordinator/src/cli.rs
+++ b/coordinator/src/cli.rs
@@ -10,7 +10,7 @@ pub fn cli(
 ) -> Result<(), Box<dyn std::error::Error>> {
     writeln!(logger, "\n=== STEP 1: CHOOSE PARTICIPANTS ===\n")?;
 
-    let participants_config = step_1(reader, logger);
+    let participants_config = step_1(reader, logger)?;
 
     writeln!(
         logger,
diff --git a/coordinator/src/step_1.rs b/coordinator/src/step_1.rs
index 7ef730c9..531f60ad 100755
--- a/coordinator/src/step_1.rs
+++ b/coordinator/src/step_1.rs
@@ -10,10 +10,13 @@ pub struct ParticipantsConfig {
 }
 
 // TODO: needs to include the coordinator's keys!
-pub fn step_1(reader: &mut impl BufRead, logger: &mut dyn Write) -> ParticipantsConfig {
-    let participants = choose_participants(reader, logger).unwrap();
+pub fn step_1(
+    reader: &mut impl BufRead,
+    logger: &mut dyn Write,
+) -> Result<ParticipantsConfig, Error> {
+    let participants = choose_participants(reader, logger)?;
     print_participants(logger, &participants.participants);
-    participants
+    Ok(participants)
 }
 
 fn validate(
@@ -21,10 +24,10 @@ fn validate(
     key_package: PublicKeyPackage,
     id_list: &[Identifier],
 ) -> Result<(), Error> {
-    if key_package.signer_pubkeys().contains_key(&id) {
-        return Err(Error::DuplicatedIdentifier);
+    if !key_package.signer_pubkeys().contains_key(&id) {
+        return Err(Error::MalformedIdentifier);
     }; // TODO: Error is actually that the identifier does not exist
-    if !id_list.contains(&id) {
+    if id_list.contains(&id) {
         return Err(Error::DuplicatedIdentifier);
     };
     Ok(())
@@ -81,3 +84,81 @@ pub fn print_participants(logger: &mut dyn Write, participants: &Vec<Identifier>
         writeln!(logger, "{}", serde_json::to_string(p).unwrap()).unwrap();
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use std::collections::HashMap;
+
+    use frost::{
+        keys::{PublicKeyPackage, VerifyingShare},
+        Error, Identifier, VerifyingKey,
+    };
+    use frost_ed25519 as frost;
+    use hex::FromHex;
+
+    use crate::step_1::validate;
+
+    const PUBLIC_KEY_1: &str = "fc2c9b8e335c132d9ebe0403c9317aac480bbbf8cbdb1bc3730bb68eb60dadf9";
+    const PUBLIC_KEY_2: &str = "2cff4148a2f965801fb1f25f1d2a4e5df2f75b3a57cd06f30471c2c774419a41";
+    const GROUP_PUBLIC_KEY: &str =
+        "15d21ccd7ee42959562fc8aa63224c8851fb3ec85a3faf66040d380fb9738673";
+
+    fn build_pub_key_package() -> PublicKeyPackage {
+        let id_1 = Identifier::try_from(1).unwrap();
+        let id_2 = Identifier::try_from(2).unwrap();
+
+        let mut signer_pubkeys = HashMap::new();
+        signer_pubkeys.insert(
+            id_1,
+            VerifyingShare::deserialize(<[u8; 32]>::from_hex(PUBLIC_KEY_1).unwrap()).unwrap(),
+        );
+        signer_pubkeys.insert(
+            id_2,
+            VerifyingShare::deserialize(<[u8; 32]>::from_hex(PUBLIC_KEY_2).unwrap()).unwrap(),
+        );
+
+        let group_public = VerifyingKey::from_hex(GROUP_PUBLIC_KEY).unwrap();
+
+        PublicKeyPackage::new(signer_pubkeys, group_public)
+    }
+
+    #[test]
+    fn check_validate() {
+        let id_1 = Identifier::try_from(1).unwrap();
+        let id_2 = Identifier::try_from(2).unwrap();
+
+        let id_list = [id_1];
+        let key_package = build_pub_key_package();
+
+        let validated = validate(id_2, key_package, &id_list);
+
+        assert!(validated.is_ok())
+    }
+
+    #[test]
+    fn check_validation_errors_for_missing_identifiers() {
+        let id_1 = Identifier::try_from(1).unwrap();
+        let id_2 = Identifier::try_from(2).unwrap();
+        let id_3 = Identifier::try_from(3).unwrap();
+
+        let id_list = [id_1, id_2];
+        let key_package = build_pub_key_package();
+
+        let validated = validate(id_3, key_package, &id_list);
+        assert!(validated.is_err());
+        assert!(validated == Err(Error::MalformedIdentifier))
+    }
+
+    #[test]
+    fn check_validation_errors_for_duplicate_identifiers() {
+        let id_1 = Identifier::try_from(1).unwrap();
+        let id_2 = Identifier::try_from(2).unwrap();
+
+        let id_list = [id_1, id_2];
+        let key_package = build_pub_key_package();
+
+        let validated = validate(id_1, key_package, &id_list);
+        assert!(validated.is_err());
+        assert!(validated == Err(Error::DuplicatedIdentifier))
+    }
+}